Electronic engineering is a technical field of transmitting, receiving, or processing signals. When undesirable signals are interfused with desired signals in signal transmission, filters, especially analog filters, are used to filter out the desired signals. Before transmitting the signals to a digital signal processing circuit, the analog filters can filter out only the signals containing desired information.
Since the cut-off frequency of a filter in an integrated circuit will be changed due to temperature variation and engineering variation, a cut-off frequency correction circuit is necessary. The cut-off frequency of the filter is dependent on a time constant of an integrator. The time constant is changed all the time due to temperature variation and engineering variation. The cut-off frequency correction is able to make the time constant be a constant value.
In an analog filter, the accuracy of the cut-off frequency is an important factor in acquiring better signal quality. When the filter is implemented by semiconductors, the cut-off frequency will be distorted ±25% at most due to engineering variation and working conditions. A Gm-C filter utilizes a phase-locked loop circuit (PLL) to adjust the cut-off frequency. In another aspect, to adjust the cut-off frequency, an active-RC filter utilizes a resistor array and/or a capacitor array to alter resistance, capacitance, or both of the resistance and capacitance.
FIG. 1 is a diagram illustrating a general structure of a resistor array for adjusting the cut-off frequency of an analog filter. FIG. 2 is a diagram illustrating a general structure of a capacitor array for adjusting the cut-off frequency of an analog filter.
When the filter is manufactured for an experimental purpose, the cut-off frequency of the filter is sometimes adjusted by manually inputting a code from external to the chip. However, when the filter is for commercial use, it is necessary to add a cut-off frequency circuit capable of adjusting a distorted cut-off frequency to the original cut-off frequency inside the chip.
Nowadays, many kinds of technical schemes have been provided for the cut-off frequency correction circuits.
FIG. 3a is a diagram illustrating a structure of a cut-off frequency correction circuit utilizing a digital-dynamical link library (DLL). FIG. 3b is a timing chart of the cut-off frequency correction circuit in FIG. 3a. 
Please refer to FIG. 3a and FIG. 3b. A clock signal (VIN) is applied to a passive filter consisted of resisters and capacitors. A comparator (CP) outputs a clock signal (D) according to the characteristics of the passive filter. The frequency of the clock signal (D) and the frequency of the clock signal (VIN) are the same, but a time delay (T1) exists therebetween. When there are no temperature variation and engineering variation, the resistance and capacitance of the passive filter is maintained at normal values, the rising edge of a LATCH CLK signal inputted from a delay generator to a latch is consistent with the rising edge of the clock signal (D) outputted from the comparator (CP). When the temperature variation and engineering variation cause the resistance and capacitance of the passive filter to be greater or smaller than the normal values, the rising edge of the clock signal (D) outputted from the comparator (CP) will lead or lag the rising edge of the LATCH CLK signal outputted from the delay generator. The difference between the rising edge positions of the clock signal (D) and the LATCH CLK signal is transmitted to an UP/DN counter. The UP/DN counter increases or decreases a bit of a resistor code each time to alter the resistance of the passive filter until the rising edge of the LATCH CLK signal outputted from the delay generator is consistent with the rising edge of the clock signal (D) outputted from the comparator (CP), and thereby the cut-off frequency is corrected.
However, in this technical scheme, the above processes need to be repeated over and over again until the desired cut-off frequency is reached. Therefore, the correction circuit would take a long time to correct the cut-off frequency.